Acoustically absorbent conduit



May 12, 1964 J. J. BARUCH ACOUSTICALLY ABSORBEINT CONDUIT 4 Sheets-Sheet 1 Filed May 27, 1955 F G. 2 INVENTOR.

JORDAN J. BARUCH BY fil 2 a" ,4

y 12, 1964 J. J. BARUCH ACOUSTICALLY ABSORBENT CONDQIT 4 Sheets-Sheet 2 Filed May 27, 1955 INVENTOR.

JORDAN J. BARUCH M M m 1964 J. J BARUCH ACOUSTICALLY ABSORBENT CONDUIT 4 Sheets-Sheet 5 Filed May 27, 1955 FIG. 4

INVENTOR. JORDAN J. BARUCH BY ATTORNEYS y 1964 J. J. BARUCH ACOUSTICALLY ABSORBENT CONDUIT 4 Sheets-Sheet 4 Filed May 27, 1955 8G SSO'! NOLLHBSNI INVENTOR.

JORDAN J. BARUCH wwL ATTORNEYS 3,132,717 Patented May'12, 1964 3,132,717 ACOUSTICALLY ABSORBENT CONDUI Jordan J. Baruch, Newton, Mass, assignor to BoltBeranek and Newman Inc., Cambridge, Mass, a corporation of Massachusetts Filed May 27, 1955, Ser. No. 511,680

I 28 Claims. (Cl. 181-5 The present invention relates to acoustic absorbers, and, more particularly, to acoustic absorbers of the type utilizing substantially helical paths for the travel of acoustic energy accompanying the flow of a fluid medium.

It has heretofore been proposed to absorb acoustic energy accompanying the flow of a fluid medium, such as air, by passing the fluid medium along a conduit in which may be disposed a helical member that forces the fluid medium to follow a helical path through the conduit. Such devices have been proposed,'for example, for use in automobile and other muffler exhausts. The very-low acoustic frequencies, such as those well below 60 or 70 cycles per second, are attenuated to a moderate degree by such helical mufi lers since longwavelength acoustic energy can not travel unimpeded along the circuitous helical path. Intermediate and high acoustic frequencies,

on the other hand, are not appreciably attenuated by such a device since, unlike the low-frequency acoustic energy, their shorter wavelength permits travel by reflections along the helical path. The intermediate and high acoustic frequencies thus travel through and, out the open end of the conduit relatively unabsorbed.

An .object of the present invention, thereof, is to provide a new and improved acoustic absorber that is particularly adaptedto attenuate or absorb, with a high degree ofkattenuation or absorption, frequencies above about 100 cycles per second, and including, in particular, the intermediate and the highaudio acoustic frequencies.

A further object is to provide a novel acoustic absorber of the helical type that is adapted toproyide' the intermediate and high frequency acoustic-energy absorption that has heretofore been unattainable with prior-art helical y muiilers v Other and further objects will beexplainedhereinafiter and will be more particularly pointed out in the appended 7 claims.

, with the present invention;

FIG. 2 is a similar-view of a modification;

FIG.u3 is a similar view, upon an enlarged scale,-of still a further modification; I r 1 FIGS. 4 and 5 are views respectivelysimilar'to FlG.

. 1 and 3, but illustratingthe helical members as provided with soundsabsorbing material; and I FIG. 6 is a. graph presenting experimentally obtained" I performance characteristics of the absorber of the present invention and the-component parts constituting the same. Refer-ring to'FlG; 1, a conduit, tube, duct or passageway is shown at 1, adapted to pass; a fluid medium,such

as air, flowing in the direction of the dash-line arrows.

The conduit 1 may be a pontionof a inufi'lenan air-conditioning duct, an aircraft engine test cell, or any other desired fluid-carrying passagewayin which it is desired to muffle the acoustic energy that may accompany the allow of the fluid medium therethrough.

in accordance. with the present invention, the conduit 1 itself is rendered absolptive' of acoustic energy, as by covering the inner walls with an acoustically absorbent lining 3. The lining 3 may be constituted'of felt, ifock- 7 W001, fiberglass or any other desired sound-absorptive material. Disposed within the conduit 1 are a plurality of blades or blade sections 5, 7, 9, mounted upon a rod support .11 that is held in position, preferably along the axis of the conduit 1, by one or more mounting rings 13,

each having a plurality of struts 15 spaced from one another to permit the flow of the fluid medium through the rings. Though the conduit is shown of preferably circular cross-section, conduits of other cross-sectional configurations may, of course, also be utilized. Each of the blades 5, 7 and 9 is disposed transversely of and at an acute angle to the longitudinal axis of the conduit 1 and each overlaps the adjacent blades to provide a relatively short substantially helical path between the blades 5, 7,

9. The blades are of sufficient transverse width that they extend substantially to the absorptive lining 3 in order that the helical path'formed between the blades may be peripherally bounded by the absorptive lining 3 of the conduit. Thoughthe blades are shown in the drawing as spaced from the lining it for purposes of clarity of illustration, in actual practice the bladesmay touch or almost touch the lining 3 so that there is no line-of-sight path for the travel of the fluid medium. A

fconduits have, however, been previously utilized for soundabsorption purposes, and it has heretofore been known that such dtvices, while providing negligible absorption to very low frequencies and to high audio frequencies, do

absorb part ofthe intermediate audio frequency spectnuni. It has not, therefore, previously been considered advan- 'it'ageous to combine helical members with'sound-absorptive conduits, or conduits having linings of sound-absorp t-ive material, since, from the above-stated characteristics, it would not be expected that the combination could possibly produceabsorption over all of theintermediate sound spectrum and over the high audible soundfrequency spectrum.

It has beendiscovered, however, that a very unexpected and highly beneficial phenomenon takes placewhen-the helical member is combined with a conduit having an acoustic lining that substantially terminates at the periphery of the helical device. Further experimental details relating to this unexpected phenomenon will later be discussed in connection with other embodiments of the in vention. It sufiices forpresent purposes to state that complete intermediate-frequency-spectrumand high audible-frequency-spectrum attenuation is thereby achieved.

The exact nature of this phenomenon that gives riseto this unexpected high-intermediate-frequency and highfrequency absorption is not fully understood, but it is known to reside in a cooperative effect between the production of reflections of the sound energy'between'the "blades 5, 7 andl9 definingthe helical path, and absorption 'of the reflected energy that is directed toward the periphery of the blades; that is, toward the acoustical-absorbent "lining 3 peripherally bounding the helical path. *By way of illustration, in a sound-absorption test conducted with a conduit 1 of about one foot inner diameter having a fiberglass lining 3 about two inches thick and three stag- .gered or over-lapping blades 5, .7, 9, providing a helical path extending approximately six inches along the axially disposed support 11, there was produced more than twenty-five decibels of attenuation of the acoustic energy accompanying the flow of air through the conduit over the complete intermediate and the high audible frequency spectrum.

The use of blades 5, 7, 9, etc. defining the helical path is, of course, far preferable to 'bafiies and other prior-art devices, since the fluid medium is forced along a smoothcurved helical path having no sharp bends, and the acousticenergy is not subject to the diffraction and other deleterious effects of such prior-art baffle systems. In addition, a much shorter path can be utilized'to obtain the desired attenuation and less fluid pressure drop is produced as the fluid medium passes through the helical path, as

contrasted with the path provided by bafile systems.

While the device of FIG. 1, as before stated, may be utilized in any desired conduit where it is desired to absorb acoustic energy, it is of particular utility in the testing of turbo-jet engines and the like where pressure losses can be tolerated.

While the helical path of FIG. 1 is very short and of very steep pitch, the blades 5, 7, 9, etc. may be spaced farther apart axially along the support 11, as shown in FIG. 2, to provide a helical path of less pitch and of longer length, along the conduit 1. The arrangement shown in FIG. 1 has the advantage that the helical device occupies very little space in the conduit 1. V The system of FIG. 2, on the other hand, has the advantage that the high-frequency acoustic energy will not undergo successive trapping reflections between the blades 5, 7, 9, etc. as may occur to some extent when the blades are closer together.

If desired, moreover, the blades need not assume the form of separate sections of helices, as in FIGS. 1 and 2,

but they may be welded together or otherwise formed as a continuous unit to define a continuous helix, or more accurately, helicoid, as shown in FIG. 3. The path followed by the fluid medium carrying the acoustic. energy is illustrated by the dash-lines between the successive blades or blade sections of the continuous helicoid 5, 7, 9, etc. of

'FIG. 3.

The performance of a system constructed in accordance with FIG. 3 is plotted in the solid-line curve A of FIG. 6.

The ordinate of FIG. 6 represents insertion loss or attenuation in units of decibels and frequency is plotted in cycles per second along the abscissa. These results were experimentally obtained witha conduit 1 about one foot in iner diameter, a fiberglass lining 3 of about four inches thickness, and a metal helical member 5, 7, 9, etc. of two full helical turns covering about two feet along the axis of the conduit 1. It will be observed that in the intermediate-frequency region of from about 400 to about 1,000

cycles, attenuation ranging from about 22-to about 36 decibels is produced; that greater attenuation is produced .vided with the above-mentioned helical member 5, 7, 9,

etc., operating as in prior-art helical mufllers. It will be observed that negligible attenuation is produced throughout the range of the intermediate and high acoustic frequencies. The dash-dot curve C illustrates the experimentally obtained attenuation characteristics of an acoustically-ab'sorbent lined conduit 1, 3 of the above dimensions, and demonstrates attenuation of a narrow band of intermediate frequencies but only a slight degree of attenuation of the low and high acoustic frequencies.

'From theoretical considerations, therefore, one would expect, upon a combination of the helical member'with the lined conduit, that one would get the summation of the characteristics B and C, namely the attenuation curve D, having a low degree of high-frequency attenuation. Curve A demonstrates, however, as before explained, an entirely unexpected characteristic producing far superior intermediate-frequency attenuation over all intermediate frequencies and a very great degree of attenuation of the high frequencies. The present invention can therefore be utilized to solve intermediate and high-frequency attenuation problems that neither the lined conduits nor the helical mufilers of the prior-art have been able to solve It does so, moreover, while making use of the advantages of a helical absorber construction.

In order further to increase the degree of absorption,

the helical blade members 5, 7, 9, etc. may be provided with layers or coverings of sound-absorbing material. The absorber of FIG. 1 is therefore illustrated in FIG. 4 with the blades 5, 7, 9 provided with acoustically absorbent layers. The blade 9' is shown provided along its left-hand surface with a layer or covering 19- of fiberglass, or any other acoustic-absorbent material, preferably faced by a metallic perforated facing 29. The blade 7 is similarly shown provided with an acoustic-absorbent layer or covering 17 faced by a perforated facing 27. The

blade 5 is similarly provided with an absorbing layer, not shown, and a perforated facing 25. The perforated facings Q5, 27, 29 provide a smooth stream-lined surface for the flow of the fluid medium in the helical path defined by the blades 5, 7, 9, and the perforations permit the acoustic energy accompanying the flow of the fluid medium'to be absorbed by the sound-absorbing layers or coverings 17, 19, etc. The layers 17, 19, etc. may be of wedge shape, as shown in FIG. 4, or they may be of substantially uniform thickness, as more particularly illustrated in the embodiment of FIG. 5. FIG. 5 is similar to :FIG. 3 except that the blades 5, 7, 9, etc. are shown provided with the before-described acoustical absorbent coverings or layers 17, 19, etc. faced by the perforated facings 27, 29, etc. The perforated facings may be of the expanded-metal or other types described in my oo-pending application, Serial No. 302,545, filed August 4, 1952, now U .8. Patent No. 2,759,554 patented August 21, 6, for Acoustic Absorber, and the sound-absorbing material 17, 19, etc. may also be of the nature of the sound-absorbing'maten'al therein described.

Instead of employing a conduit 1 as of metal, ceramic or other sound-conducting material and providing an absorptive lining 3, moreover, the conduit may, in some cases, be fabricated from sound-absorptive material.

It is apparent from the foregoing description, particularly with reference to the curves of FIGURE 6, that the lining material must be capable of providing high acoustic absorption in the intermediate range of audible acoustic frequencies, that is frequencies of the order of 1000 c.p.s., and, by virtue of the cooperation of ube lining and the helical path, high acoustic absorption of the higher audible acoustic frequencies as well. Materials such as the aforementioned felt, rock wool, and fiberglass, of the order of several inches thick provide such absorption by means of losses accompanying the passage of acoustic energy through a myriad of irregular, minute interstices, and will be referred to generically as fiberglass like.

Further modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention, as defined in the appended claims;

What is claimed is:

#1. An lzroustically absorbent conduit having means comprising acoustically impervious blades having an acoustically absorbent covering and mounted within and along the conduit to provide a substantially helical path therebetween peripherally bounded by the conduit, with the said path the only path for the travel of acoustic energy along the conduit.

2. An acoustically absorbent conduit having means comprising acoustically impervious blades mounted withi in and along the conduit to provide a substantially helical path thcrebetween peripherally bounded by the conduit, the blades carrying an acoustically absorbent layer 7 covered by a perforated facing,-wit-h thesaid paththe ,only path for the travel of acoustic energy along the conduit.

3. An acoustic absorber havingin combination, laconduit, an acoustically absorbent lining disposed along the ing by the acoustically absorbent lining, the helical-member being provided with an acoustically absorbent layer covered by a perforated facing. p I I p 5. An acoustic absorber having, in combination, a conduit, an acoustically absorbent lining disposed along the inner walls of the conduit, and means comprising acoustically impervious blades having an acoustically absorbent covering and mounteduwithin and along the conduit to provide a substantially helical path'therebetween per-ipherally bounded by the'sa-id acoustically absorbent lining, with the said path the only path for the travel of acoustic energy along the conduit. 6. An acoustic absorber having, in combination, aconnun, an acoustically absorbent lining disposed along the inner walls of the conduit; and means comprising-acoustically impervious blades-mounted within and along the :conduit toprovidea substantially helical path] therebetween peripherally-bounded by the said acoustically absorbent lining, the blades carrying m acoustically absorjbent layer covered by a perforated facing, with the said paththe only path for the travelof acoustic energy along the conduit.

7. An acoustically absorbent conduit having means comprising acoustically impervious blades having an acoustically absorbent covering and mounted within and substantially throughout the length of the conduit to provide a substantially helical path therebetween peripherally bounded by the conduit, with the said path the only path for the travel of acoustic energy along the conduit.

8. An acoustically absorbent conduit having high acoustic absorption for intermediate audible acoustic frequen- 0 cies and having means comprising separate acoustically reflective impervious blades mounted transversely along between adjacent blades a ripherallybounded by the conduit, the blades carrying an acoustically absorbent layer covered by a perforated e substantially helical path pefacing, with the said path the only path for the travel of acoustic energy along the conduit. I

11. An acoustically absorbent conduit having means comprising acoustically impervious blades having an acoustically absorbent'covering and mounted within and along the-conduit to provide a substantially helical path therebetween peripherally bounded by the conduit, and

input and ,output apertures disposed at the ends of the conduit and of substantially the same cross-dimension as that of the blades, with the said patht-he only path for the travel of acoustic energy along the conduit.

12. An acoustic absorber having, in combination, a conduit, a smooth acoustically absorbent lining continuously dispersed along the inner walls of the conduit, and means comprising acoustically impervious'blades carrying a smooth acoustically absorbent layer mounted within and along the conduit to provide a substantially smoothlined helical path therebetween peripherally bounded by the said smooth acoustically absorbentlining, with the said path the only path for the travel of acoustic energy along the conduit 13. A sound mufl'ler for gases effective for attenuating sound throughout a wide band of audible frequencies comprising a tubular duct, a porous acoustically absorbent but at an acute angle to the axis of the conduit with each blade partially overlapping the adjacent blade or blades to provide a substantially helical path therebetween peripherally bounded by the conduit, with the said path the only path for the travel of acoustic energy along the conduit.

9. An acoustically absorbent conduit having high acoustic absorption for intermediate audible acoustic frequencies and having means comprising a support mounted along the axis oi the conduit and carrying therealong separate acoustically reflective impervious blades disposed transversely of and at an acute angle to the axis with each blade partially overlapping the adjacent blade or blades to provide a substantially helical path therebetween peripherally bounded by the conduit, with the said path the only path for the travel of acoustic energy along the conduit.

10. An acoustically absorbent conduit having means comprising a support mounted along the axis of the conduit and carrying therealong connected acoustically impervious blades disposed transversely of and at an acute angle to the axis to form a continuous helix defining cal.

fiberglass-like lining means at least substantially several inches thick on the inner face of the duct for substantially absorbing intermediate audible frequencies in said band,

and an imperforate acoustically reflective baflle peripherally terminating at the lining, having longitudinal axis extending lengthwise of the ,duct, having an axial t-Wist execeding onehalf turn, and substantially occluding said duct with respect to axial acousticflow therethrough.

14. The muffler of claim 13, wherein'said baflleis heli- 15. The muffler of claim 13, wherein said duct has in- ".put and output pipes disposed at its ends, respectively,

fiberglass-like material at least substantially several inches thick for providing high acoustic absorption of intermediate audible acoustic frequencies, and having means comprising ian acoustically reflective impervious blade mounted within and along the conduit for providing a substantially helical path peripherally bounded by the conduit, and for substantially occluding said conduit with respect to axial acoustic flow therethrough.

19. An acoustic absorber having, in combination, a conduit, an acoustically absorbent lining of fiberglass-like material at least substantially several inches thick disposed along the inner walls of said conduit for substantially absorbing intermediate audible acoustic frequencies of the order of 1000 c.p.s., whereby said frequencies are greatly attenuated, and means comprising an acoustically reflective impervious member mounted within and along the conduit for providing a substantially helical path peripherally bounded by said lining and for substantially occluding said conduit with respect to axial acoustic flow therethrough, whereby the higher audible acoustic frequencies are greatly attenuated.

20. The absorber of claim 19, said member extending substantially throughout the length of said conduit.

21. A conduit having an acoustically absorbent wall of fiberglass-like material at least substantially several inches thick for providing high acoustic absorption of intermediate audible acoustic frequencies, and having means com- 18. A conduit having an acoustically absorbent wall of prising connected acoustically reflective impervious blades mounted transversely along but at an acute angle to the axis of the conduit to form a continuous helix defining between adjacent blades a substantially helical path periphcral-ly bounded by the conduit and completely occluding the conduit with respect to axial acoustic flow therethrough.

22. The conduit of claim 21, said blades being carried by a support mounted alng the axis of said conduit.

23. The conduit of claim 22, said conduit having mounting-ring means provided with a plurality of spaced struts for securing said support along the axis of said conduit.

' 24. An acoustically absorbent conduit having means comprising acoustically impervious blades within and along the conduit for providing between said blades a substantially helical path peripherally bounded by the conduit andlfor substantially occluding said conduit with respect to axial acoustic flow therethrough, and input and output pipes connected to the ends of the conduit and of substantially the same inner cross-dimension as that of the helical path.

25. The conduit of claim 24, said means comprising connected blades mounted transversely along but at an acute angle to the axis of said conduit to form a continuous helix, said pipes being axially aligned with said 'blades.

26. The conduit of claim 24, further having a support mounted along the axis of the conduit and carryingsaid blades.

27. Anacoustic absorber having, in combination, a conduit, an [acoustically absorbent lining disposedalong the innerwalls of said conduit, and means comprising acoustically impervious blades mounted withinand along the conduit for providing between said blades a substan tially helicalpathperipherally bounded by said acoustically absorbent lining and for substantially occluding said conduit with respect to axial acoustic flow therethrough, the conduit having input and output pipes at ,its endsof substantially the same inner cross-dimension-as said helical path.

28. A conduit havinga smooth continuous acoustically absorbent inner lining of fiberglass-like material at least substantially several inches thick for substantially absorbing intermediate audible acoustic frequencies and having means comprising acoustically reflective impervious'blades mounted Within and substantially throughout the length of the conduit for providing between said blades a substantially helical path peripherally bounded by the smooth continuous lining .and for substantially occluding said conduit with respect to axial acoustic flow therethrough.

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18. A CONDUIT HAVING AN ACOUSTICALLY ABSORBENT WALL OF FIBERGLASS-LIKE MATERIAL AT LEAST SUBSTANTIALLY SEVERAL INCHES THICK FOR PROVIDING HIGH ACOUSTIC ABSORPTION OF INTERMEDIATE AUDIBLE ACOUSTIC FREQUENCIES, AND HAVING MEANS COMPRISING AN ACOUSTICALLY REFLECTIVE IMPERVIOUS BLADE MOUNTED WITHIN AND ALONG THE CONDUIT FOR PROVIDING A SUBSTANTIALLY 